Mesotheliomas, malignant tumors of the serous lining of the body cavities, are notoriously difficult to differentiate from other tumors involving these cavities. An antiserum specific for mesothelial cells was developed by immunizing rabbits with cultured mesothelial cells and appropriate absorptions. The antibody has been used in indirect immunofluorescence and immunoperoxidase staining assays to differentiate mesotheliomas from other malignant tumors. The antibody described above was of low titer and could not be used to isolate mesothelial cell specific antigen by affinity chromatography. A large batch of cultured normal mesothelial cells will be extracted with PbS containing 0.1% NP 40 and subjected to negative affinity chromatography with antisera to fetal calf serum, human serum, and colon, and breast carcinoma tissues. The partly purified mesothelial antigen will be used to raise a batch of high titered antibody to mesothelial cells. The antibody will be used to isolate the mesothelial antigen. The high titered antibody and the isolated pure mesothelial cell antigens will then be employed to develop a quantitative immunoassay for mesothelial antigen. The assay will be used as a non invasive diagnostic tool for mesotheliomas as well as for quantitation of mesothelial cell mass.